The present invention relates to an electrophotographic image forming apparatus of the type using single-ingredient type high resistance toner, and a developing roller or similar toner conveyor roller included in the apparatus.
An electrophotographic image forming apparatus operable with single-ingredient type high resistance toner is advantageous in that, e.g., a developing device and therefore the entire apparatus is small size, and in that maintenance is basically needless. The apparatus can be further miniaturized if an image carrier is implemented as a photoconductive drum. However, with a photoconductive drum, it is difficult to uniformly charge the toner due to the low reliability of the drum.
Two different developing systems using the above toner are available in the imaging art. A first developing system uses an elastic developing roller. The problem with this system is that toner particles charged to the polarity opposite to the expected polarity appear and deposit on and contaminate the background of a sheet. A second developing system has an intermediate roller between a developing roller and a photoconductive element in order to eliminate the above problem. Specifically, the second system protects a sheet from background contamination because when the toner is transferred from the developing roller to the intermediate roller, the toner particles of opposite polarity are left on the developing roller.
Japanese Patent Publication No. 6-64366 proposes an arrangement wherein charged toner of the kind described is conveyed by first conveying means to second conveying means, transferred to the second conveying means by an electrical force, and then conveyed to an image carrier by the second conveying means. The first and second conveying means are implemented as a roller and a belt, respectively, while the image carrier is implemented as a drum. The belt is held in contact with the roller and drum. On the other hand, Japanese Patent Laid-Open Publication No. 6-175477 teaches a toner conveyor roller formed of a soft material and intervening between a hard photoconductive drum and a conveyor roller. This kind of scheme further miniaturizes the apparatus.
In the first developing system, it is necessary to form a layer of uniformly charged toner on the developing roller. The amount of toner must be small enough for the toner to be uniformly charged. If the linear velocity ratio of the toner conveyor roller to a photoconductive element is excessive, the returning force of the toner is excessively increased at the nip while the background is contaminated by, e.g., friction between the photoconductive element and the toner. Further, even when a doctor blade is held in contact with the developing roller in order to charge the toner by friction while regulating the amount of toner, or even when charge is injected into the toner, the amount of toner (for a unit area of the developing roller) which can be uniformly charged is limited. Hence, although the toner may be uniformly charged, the amount of toner deposited on the developing roller and the amount of toner required on the photoconductive element are not always identical. In addition, if the above limit is exceeded, there increases the ratio of toner particles of opposite polarity and uncharged particles to the entire particles. Therefore, at the present stage of development, the apparatus design is required to balance the amount of toner on the developing roller and the linear velocity ratio of the developing roller to the photoconductive element. This eventually does not always result in desirable images.
The above is also true with the second developing system or two-step developing system. Specifically, the developing roller and intermediate roller are each driven at a particular linear velocity. In addition, the linear velocity of the intermediate roller and that of the photoconductive element are often slightly different from each other. Therefore, a stress acts between the developing roller and the intermediate roller and between the intermediate roller and the photoconductive element. Particularly, when the developing roller and intermediate roller are moved in opposite directions at a position where they contact each other, the stress acting there is great. As a result, if the intermediate roller is coated with a dielectric material, the dielectric layer is apt to wear or come off. The dielectric layer often comes off at the end portions thereof. Once the dielectric layer comes off, it is likely that the underlying conductive layer exposed to the outside contacts the photoconductive element, resulting in a current leak. The current leak destroys the dielectric layer and makes the roller unusable.
In the first and second developing systems, the developing roller and intermediate roller (collectively referred to as a toner conveyor roller hereinafter) each contacting the photoconductive element are elastic or soft. While the toner conveyor roller must be semiconductive, it is most desirable that the roller be provided with a dielectric outermost layer and a semiconductive or conductive underlying layer in order to prevent current from leaking at the position where the roller contacts the photoconductive element. For this reason, the conventional toner conveyor roller has a conductive elastic layer, and a dielectric layer formed on the conductive layer. However, the dielectric layer is extremely thin. When the toner conveyor roller and photoconductive element contact each other, the insulation distance between them decreases. As a result, current is apt to leak between the edge of the end portion of the conductive layer and the photoconductive element. Further, the end portion of the conductive layer often bulges out, depending on the production method. When the roller in this condition is coated with a dielectric material, the dielectric layer is reduced in thickness at the end portion thereof. If this kind of roller is held in contact with the photoconductive element, the current leak is aggravated.
As stated above, when use is made of an elastic toner conveyor roller, current leaks due to the peeling of the dielectric layer which is ascribable to a stress acting on the roller, or leaks between the edge of the conductive layer and the photoconductive element on the contact of the roller with the element. The leak not only results in defective images including images with contaminated background, but also destroys the dielectric layer of the roller and that of the photoconductive element and makes them practicably unusable.